What do sea grasses in the Florida Bay, blue crabs in the Chesapeake and sea otters in the North Pacific have in common? While scientists have tried to pin the blame for their decline on causes from pollution to climate change, a new international study concludes that denizens of the deep are disappearing due to one cause: overfishing. Centuries of overharvesting, the researchers report in the current issue of the journal Science, have in turn left organisms vulnerable to disease, pollution and climate change. "We already knew about commercial fishing, but we were stunned at how much humans have affected these systems all the way back through history," says marine biologist Bob Warner of the University of California, Santa Barbara.

Using ancient ecological, archeological and historical data, the scientists found that people made a dent in marine-animal populations long before modern times. The remains of trash piles show that Native Americans began harvesting green sea turtles from the Caribbean 2,000 years ago. Then, in the 1700s, European colonists hastened sea turtles' demise, so that today only 10 percent of the original population of about 16 million sea turtles remains. That in turn has hurt sea grasses in the Florida Bay: without sea turtles to crop the plants, seagrass beds are overcrowded and the plants are dying from disease. "We've always presumed that these diseases were a result of the crap we're dumping in the ocean or the changes in water quality," says biologist Jim Estes of the University of California, Santa Cruz. Overfishing also hurt blue crabs in the Chesapeake: oyster beds that used to shelter baby crabs were dredged away. And sea otters in the North Pacific seem to have been decimated by killer whales, which were left without their traditional food of fin and sperm whales after the great whale hunts of the past century.

All may not be lost, however. Protecting marine reserves from fishing could help restore the stocks, as long as tough rules protect water quality. We've been trawling the oceans for centuries. But we may be the last generation to do so if we don't protect what's left.